1. Field of the Invention
The present invention relates to image forming processing for forming an image on a recording medium using a black color material and a plurality of chromatic color materials.
2. Description of the Related Art
The technique capable of realizing a highly developed color by enlarging the color gamut (i.e., color reproduction range) of a printer is conventionally known. As discussed in Japanese Patent Application Laid-Open No. 6-233126, it is conventionally known that particular color inks enlarging the color gamut of basic red, green, and blue colors are usable in addition to basic color inks of cyan, magenta, yellow, and black.
More specifically, it is conventionally known that a red color area of the color gamut can be enlarged by using a red ink that can reproduce a red color having a higher saturation value compared to a red color formed by overlapping a magenta dot and a yellow dot.
As discussed in Japanese Patent Application Laid-Open No. 2004-155181, it is conventionally known that a developed color formed by overlapping a yellow dot and a cyan dot in this order is different from a developed color formed in the opposite order (i.e., in order of a cyan dot and a yellow dot).
Further, it is conventionally known that the color gamut can be enlarged by designating the order of yellow and cyan in image formation when the color to be developed is reproducible only when the yellow and cyan dots are overlapped in this order. Similarly, the color gamut can be enlarged by designating the order of cyan and yellow in image formation when the color to be developed is reproducible only when the cyan and yellow dots are overlaps in this order.
As discussed in Japanese Patent Application Laid-Open No. 2005-88579, it is conventionally known that a developed color is deteriorative in properties if a red dot is overlapped with yellow and magenta dots compared to a color developed without overlapping these dots. Further, it is conventionally known that the color gamut can be enlarged by differentiating a dot layout pattern applied to red color from a dot layout pattern applied to other colors in such a way as to reduce the above-described overlapping probability, when quantized color material amount data is converted into binary data indicating formation/non-formation of a dot in relation to a predetermined dot layout pattern.
As a problem peculiar to a print product printed by a pigment inkjet printer that mainly uses pigments as coloring materials, the color gamut deteriorates at a dark portion (i.e., a low-lightness region). FIG. 1 schematically illustrates the shape of a color gamut relating to yellow hue of a print product printed by the pigment inkjet printer. In FIG. 1, the abscissa axis represents the magnitude of saturation C* and the ordinate axis represents the magnitude of lightness L* in a CIELCh color space, in which point A, point B, point C, and point D indicate colors adjacent to white, yellow, black, and color adjacent to black at the surface of the color gamut, respectively.
As illustrated in FIG. 1, the contour of the color gamut extending from yellow to black is greatly deformed inward at a dark portion (i.e., a low-lightness region), compared to a straight line connecting the point B (yellow) and the point C (black). Especially, in an area adjacent to black, the saturation does not change so largely even when the lightness changes greatly, as understood from the positional relationship between the point C and the point D. Therefore, the color gamut having a formed shape causes defective gradation (collapse) in color mapping.
In this respect, according to the techniques discussed in Japanese Patent Application Laid-Open No. 6-233126 and Japanese Patent Application Laid-Open No. 2005-88579, printers are required to have a complicated and enlarged structure due to newly added inks. Further, according to the technique discussed in Japanese Patent Application Laid-Open No. 2004-155181, the color gamut of intermediate lightness is enlarged in the hue extending from yellow to cyan via green. However, the color gamut of the dark portion (low-lightness region) extending from yellow to black cannot be enlarged.